Conventionally as shown in FIGS. 7A and FIG. 7B, in order to stack sheets of corrugated cardboard, paper, plastic film, foil and other various material cut by a sheet cutting machine 82 into the stacking zone, sheets 86 are transferred by a high speed conveyor 84 and their transfer speed is reduced by an overlap conveyor 88 (a low speed conveyor) having a speed difference from the high speed conveyor. The sheets, whose speed is reduced, are then overlapped on the overlap conveyor one after another, are transferred by the overlap conveyor, and are stacked into the stacking zone, lot by lot overlapped.
Many kinds of overlap conveyors are used. For example, sheets travelling on the high speed conveyor are nipped, slowed down and overlapped on the overlap conveyor by "pressing whiskers" of the overlap conveyor, with vacuum or the like, and are then transferred on the overlap conveyor. Or, sheets transferred by the high speed conveyor are slowed down and overlapped by a "catching belt", "pressing belt", "low speed conveyor" or the like.
FIG. 7A shows an example where pressing whiskers 90 are used, and FIG. 7B shows an example where a catching belt 92 is used.
However, a system using such an overlap conveyor has various problems such that grazes are produced on surface of a sheet travelling at a high speed due to friction of "pressing whiskers", "catching belt", "pressing belt" or the like, the leading edge of sheet is folded or damaged because of high speed sheet transfer by the conveyor, and normally the leading edge of a sheet droops down or bumps against preceding sheet, especially when sheet stiffness is low.
Because cutting performance of cutting machine has recently been improved remarkably, it is required to transfer cut sheets at a high speed. Therefore, it becomes necessary to install another higher speed conveyor upstream of said high speed conveyor, resulting in lengthening a line by said higher speed conveyor, a larger installation space, and an increase of installation cost.
Furthermore, because materials to be cut by a sheet cutting machine vary to include thicker and thinner corrugated cardboard, paper, plastic film, foil and the like, it becomes impossible to solve the various problems for such various materials by a conventional sheet stacker, namely by a stacker for transferring cut sheets by a high speed conveyor, reduction of sheet speed by an overlap conveyor, overlapping the sheets, sending the sheets to the stacking zone and stacking the sheets into the stacking zone, lot by lot overlapped.
A means to remove such problems was disclosed in U.S. Pat. No. 557,439, "Tail Stopping and Knockdown Device". As shown in FIG. 8, this device has a slowing down device comprising a brush roll 103 and a low speed roll 104 between a conveyor 101 and a stacking station 102. The rolls 103 and 104 are driven by a motor 106 through appropriate gearings 105.
In this conventional technology, sheets are slowed down and sent to the stacking station 102. Their portions near the tails ends are contacted by one of two brushes 108 of the brush roll 103 and pressed to the roll 104 rotating at a low speed, when sheets pass between two rolls 103 and 104. But with this type of brush roll 103, the pressing action of brush 108 to press the sheets to the roll 104 is unstable, and the sheets could be damaged by the brushes in case of paper sheets or the like. Furthermore, the pressing positions of sheets cannot be accurately controlled, and therefore, when the pressing positions of sheets are shifted, sheets might not be stacked into the stacking orderly.